The improved photovoltaic devices disclosed herein exhibit increased operational efficiency over the prior art due to improved reflectivity of the underlying reflective layer. Also disclosed are methods for the fabrication of the improved photovoltaic devices. The present invention has particular applicability to (1) large area, thin film, amorphous photovoltaic devices wherein the active semiconductor elements thereof are deposited onto a substrate electrode as relatively thin layers which are subsequently covered by a second electrode, and (2) the fabrication of such thin film, large area photovoltaic devices from amorphous semiconductor alloys.
Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent, and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter, (2) are thicker and heavier than their amorphous counterparts, and (3) are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous," as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge deposition or vacuum deposition techniques. The silicon alloy material possesses (1) acceptably reduced concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques have been fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan and issued on Oct. 7, 1980, the disclosure of which is incorporated herein by reference. As disclosed in this patent, fluorine introduced into discrete layers of the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Unlike crystallize silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409 entitled A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 4,410,558 entitled Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723, entitled Multiple Chamber Deposition And Isolation System And Method; U.S. Pat. No. 4,492,181 entitled Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. No. 4,485,125 entitled Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; the disclosures of which are incorporated herein by reference. As disclosed in these patents, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configuration, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an i-type intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy.
Since each deposited layer of semiconductor alloy material, and especially the intrinsic layer, must be of high purity; (1) the deposition environment in the intrinsic deposition chamber is isolated, by specially designed gas gates, from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber; (2) the substrate is carefully cleansed prior to initiation of the deposition process to remove contaminants; (3) all of the chambers which combine to form the deposition apparatus are sealed and leak checked to prevent the influx of environmental contaminants; (4) the deposition apparatus is pumped down and flushed with a sweep gas to remove contaminants from the interior walls thereof; and (5) only the purest reaction gases are employed to form the deposited semiconductor materials. In other words, every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 4,949,498 issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant.
Most photovoltaic devices, having either single cell or multiple cell structures, preferably include a light reflecting back reflector for increasing the percentage of incident light reflected from the substrate back through the active semiconductor alloy material of the cells. It should be obvious that the use of a back reflector increases the amount of light which passes through the active semiconductor allow material thus increasing the amount of incident light which is converted to electricity, and increasing the operational efficiency of the photovoltaic device. However, all layers, other than the layers of semiconductor material, deposited atop the light incident surface of the substrate must be substantially transparent (to 350 nanometer to 1 micron light) so as to pass a high percentage of incident light from the anti-reflective coating atop the photovoltaic cell to the highly reflective surface of the back reflector from which it is redirected through and absorbed by the semiconductor layers.
The back reflector may be formed atop the deposition surface of the substrate if an opaque substrate is employed and may be either specular or diffuse. With either type of back reflector, light which has initially passed through the active body of semiconductor alloy material from which the photovoltaic device is fabricated without being absorbed on its initial pass, is redirected by the highly reflective material of the back reflector to pass, once again, through the photoactive layers. The additional pass results in increased photon absorption and charge carrier generation, thereby providing increased short circuit current.
In the case of specular back reflectors, wherein the highly reflective material is conformally deposited over a smooth surface, the unused light is generally redirected for one additional pass through the active body of semiconductor alloy material of the device.
The diffuse back reflectors scatter the light incident thereupon in addition to being redirected through the photoactive layers, thereby mandating that a portion of the redirected light travel at angles sufficient to cause the redirected light to be substantially confined within the photovoltaic device, i.e., achieve total internal reflection. This scattering is accomplished by either 1) a highly reflective material which is grown in a textured manner upon an underlying surface, 2) light scattering film layers disposed on top of a specular reflecting surface to scatter incident light. This internal reflection provides for lengthened photon paths through the active semiconductor ally material, thus increasing the operational efficiency of the photovoltaic device. Since diffuse back reflectors redirect light through the photoactive layers of the photovoltaic device at an angle, the photoactive layers appear thinner, thereby reducing charge carrier recombination, while still maintaining efficient charge carrier generation (improving short circuit current) and promoting charge carrier collection (improving the fill factor). Also textured back reflectors contribute to textured overlying layers of semiconductor alloy material. Textured semiconductor alloy material has a larger boundary surface area, thus making the path a charge carrier must travel to collection shorter, and therefore reducing the likelihood of recombination in the photoactive layers.
As should be apparent from the foregoing discussion, and since the purpose of a back reflector of a photoresponsive device is to redirect incident light for at least a second pass through the photoactive layers of material thereof, absorption of that incident light by the back reflector cannot be tolerated. The material employed as the back reflector cannot interact with, or diffuse into, the overlying layers of semiconductor alloy material. Additionally, the back reflector material cannot add series resistance to the photovoltaic device, nor can it be too soft, since softness decreases yield, while causing short circuit junctions, and interlayer peeling.
Accordingly, several different materials have been investigated for use as the back reflector in photovoltaic devices. Several are reviewed hereinbelow. For use as back reflectors, the most highly reflective material is silver which is characterized by an integrated reflectivity of about 98.5% at 700 nm. Aluminum is another highly reflective back reflector material commonly used in the fabrication of back reflectors. Aluminum has an integrated reflectivity of about 90% at 700 nm. Yet another highly reflective material has been proposed for use as a back reflector is copper which is characterized by an integrated reflectivity of about 97.5% at 700 nm. The last of the most commonly employed reflective materials from which back reflectors are fabricated, is stainless steel having an integrated reflectivity of about 45%. While stainless steel is not nearly as reflective (and indeed, was not described as "highly" reflective) as aluminum, silver, and copper, it has been utilized as a substrate material and hence remains a possible candidate when economic factors are taken into consideration.
Previous attempts to employ aluminum as the highly reflective material of a back reflector for a photoresponsive device, which included a body of amorphous silicon alloy semiconductor material, have been unsuccessful because of the interdiffusion problems alluded to herein above. More particularly, when the amorphous silicon alloy material is deposited upon highly reflective material fabricated from aluminum, interdiffusion of the silicon and the aluminum from the contiguous layers results. Obviously the photogenerating and photoconductive properties of the body of silicon alloy material, as well as the reflective properties of the back reflector suffer.
Prior attempts at utilizing copper to fabricate the highly reflective material of the back reflector for a photoresponsive device have, likewise, proven unsatisfactory. This can be traced to the incompatibility of copper to a subsequently deposited fluorinated silicon alloy semiconductor material. More specifically, fluorine from the body of fluorinated amorphous silicon alloy material would react with the highly reflective copper material to form a copper:fluoride compound. As with the aluminum:silicon compound discussed in the preceding paragraph, the copper:fluoride compound deleteriously affected both the reflective properties of the copper back reflector and the photogenerative and the photoconductive properties of the semiconductor material. And as with the aluminum back reflector material described in the preceding paragraph, even when copper was sandwiched between buffer layers of titanium, or tin oxide, of a thickness specifically tailored to prevent light absorption, the buffer layers were ineffective in sufficiently preventing interdiffusion in a manner which would maintain the desired photovoltaic properties of the semiconductor material and the reflective characteristics of the back reflector.
Prior attempts to use silver as the highly reflective material from which to fabricate the back reflectors for photoresponsive devices have also not been very successful despite the fact that the silver back reflective material presents no serious interdiffusion problems with regard to the semiconductor material. However, silver and silver alloys present their own particular problem when employed as a highly reflective back reflector material, i.e., silver due to its relatively soft nature tends to easily deform, particularly during processing subsequent to the deposition of the body of semiconductor material, thus causing shunts or shorts in the cell. Further, silver is expensive as compared to other back reflector materials such as for example aluminum.
Thin film amorphous semiconductor materials offer several distinct advantages over crystalline materials, insofar as they can be easily and economically fabricated by the newly developed mass production processes. Back reflectors commonly formed of the aforementioned highly reflective materials have further been employed in an attempt to provide a suitable light redirecting layer for photoresponsive devices. However, as demonstrated herein above, each of the highly reflective materials have significant shortcomings when used as back reflector materials.
Peeling and cracking (adhesion failure) of the body of semiconductor material from the deposition surface of the highly reflective back reflector is believed to be due to the interdiffusion of elements at the back reflector-semiconductor material interface. The elements from which the (1) highly reflective back reflector material, (2) substrate material, and (3) semiconductor material, as described supra, are fabricated, diffuse through the respective interfaces and strain, or otherwise deleteriously affect, the chemical bonds which exist between the materials from which the contiguous layers of the aforementioned interfaces are fabricated. The resultant bond strains and recombinations cause the body of semiconductor material to crack and peel off of the underlying back reflector. And even when the aforementioned adhesion promoting and diffusion limiting layers were employed to isolate the back reflector from the substrate and the body of semiconductor material, the silver, aluminum or copper material from which the highly reflective layers are fabricated, would agglomerate, thereby substantially inhibiting, if not totally preventing, the adhesion of the subsequently deposited back reflector.
In an attempt to alleviate these problems of diffusion, reflective layers have been covered with protective layers of materials, including metals and their oxides. However, the use of such diffusion inhibiting layers (also referred to as "buffer layers") was not totally effective. The layers had to be very thin in order to prevent their absorption of incident light. However, when made very thin, these layers could not effectively prevent interdiffusion. Alternatively, if the layers were deposited so as to prevent diffusion, they would be so thick as to absorb the incident light. It was further found that in order to increase the transparency of the buffer layer, the oxygen content must be increased but this increased the resistance of the buffer layer, thus decreasing photoefficiency of the device. Conversely, reduced resistance decreased the amount of light reflected from the surface of the back reflector.
Layers of resistive material have been utilized in photovoltaic devices to minimize defect regions, such as short circuits or other low resistance shunt paths between electrodes. Such buffer layers are disclosed in U.S. Pat. No. 4,598,306, entitled Barrier Layer for Photovoltaic Devices; and U.S. Pat. No. 4,532,372, also entitled Barrier Layer for Photovoltaic Devices. Such buffer layers are in the thickness range of 200-1500 Angstroms and have resistivities in the range of 10.sup.3 to 10.sup.8 ohm/cm. As disclosed, the barriers may be formed from magnesium fluoride or oxides, nitrides, and carbides of: indium, tin, cadmium, zinc, antimony, silicon, chromium, and mixtures thereof. These barrier layers form a continuous layer of increased resistance between the electrodes to decrease the flow of electrical current through defect regions therebetween. As disclosed specifically in the '372 patent, a continuous magnesium fluoride barrier layer of 200 Angstroms thickness may be utilized with advantage only in those cells which are subject to low level (i.e. room light) illumination.
It has been found that the presence of oxide layers proximate reflective layers can cause oxidation of the reflective layer thereby significantly decreasing its reflectivity. For example, a silver back reflector and a titanium oxide or tin oxide layer react to form silver oxide, which is black instead of transparent. The formation of silver oxide decreases the integrated reflectivity of the back reflector by as much as 15% thereby decreasing overall cell efficiency.
In summary, reflective layers of silver, aluminum or copper are frequently employed in solar cells. Furthermore, such cells often include barrier layers to prevent diffusion of reflective material into the semiconductor, since such diffusion can degrade the semiconductor. However, the buffer layers have been found to adversely react with the reflector material causing it to tarnish.
Thus, based upon the foregoing, it should be apparent that a need exists for a photoresponsive device having highly reflective back reflector, but which is free from problems attendant upon diffusion of reflector material into the semiconductor and/or oxidation of the reflector by a diffusion preventing buffer layer.